1*** Reserved memory regions ***
2
3Reserved memory is specified as a node under the /reserved-memory node.
4The operating system shall exclude reserved memory from normal usage
5one can create child nodes describing particular reserved (excluded from
6normal use) memory regions. Such memory regions are usually designed for
7the special usage by various device drivers.
8
9Parameters for each memory region can be encoded into the device tree
10with the following nodes:
11
12/reserved-memory node
13---------------------
14#address-cells, #size-cells (required) - standard definition
15    - Should use the same values as the root node
16ranges (required) - standard definition
17    - Should be empty
18
19/reserved-memory/ child nodes
20-----------------------------
21Each child of the reserved-memory node specifies one or more regions of
22reserved memory. Each child node may either use a 'reg' property to
23specify a specific range of reserved memory, or a 'size' property with
24optional constraints to request a dynamically allocated block of memory.
25
26Following the generic-names recommended practice, node names should
27reflect the purpose of the node (ie. "framebuffer" or "dma-pool"). Unit
28address (@<address>) should be appended to the name if the node is a
29static allocation.
30
31Properties:
32Requires either a) or b) below.
33a) static allocation
34   reg (required) - standard definition
35b) dynamic allocation
36   size (required) - length based on parent's #size-cells
37                   - Size in bytes of memory to reserve.
38   alignment (optional) - length based on parent's #size-cells
39                        - Address boundary for alignment of allocation.
40   alloc-ranges (optional) - prop-encoded-array (address, length pairs).
41                           - Specifies regions of memory that are
42                             acceptable to allocate from.
43
44If both reg and size are present, then the reg property takes precedence
45and size is ignored.
46
47Additional properties:
48compatible (optional) - standard definition
49    - may contain the following strings:
50        - shared-dma-pool: This indicates a region of memory meant to be
51          used as a shared pool of DMA buffers for a set of devices. It can
52          be used by an operating system to instantiate the necessary pool
53          management subsystem if necessary.
54        - vendor specific string in the form <vendor>,[<device>-]<usage>
55no-map (optional) - empty property
56    - Indicates the operating system must not create a virtual mapping
57      of the region as part of its standard mapping of system memory,
58      nor permit speculative access to it under any circumstances other
59      than under the control of the device driver using the region.
60reusable (optional) - empty property
61    - The operating system can use the memory in this region with the
62      limitation that the device driver(s) owning the region need to be
63      able to reclaim it back. Typically that means that the operating
64      system can use that region to store volatile or cached data that
65      can be otherwise regenerated or migrated elsewhere.
66
67Linux implementation note:
68- If a "linux,cma-default" property is present, then Linux will use the
69  region for the default pool of the contiguous memory allocator.
70
71- If a "linux,dma-default" property is present, then Linux will use the
72  region for the default pool of the consistent DMA allocator.
73
74Device node references to reserved memory
75-----------------------------------------
76Regions in the /reserved-memory node may be referenced by other device
77nodes by adding a memory-region property to the device node.
78
79memory-region (optional) - phandle, specifier pairs to children of /reserved-memory
80memory-region-names (optional) - a list of names, one for each corresponding
81  entry in the memory-region property
82
83Example
84-------
85This example defines 3 contiguous regions are defined for Linux kernel:
86one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
87one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB), and
88one for multimedia processing (named multimedia-memory@77000000, 64MiB).
89
90/ {
91	#address-cells = <1>;
92	#size-cells = <1>;
93
94	memory {
95		reg = <0x40000000 0x40000000>;
96	};
97
98	reserved-memory {
99		#address-cells = <1>;
100		#size-cells = <1>;
101		ranges;
102
103		/* global autoconfigured region for contiguous allocations */
104		linux,cma {
105			compatible = "shared-dma-pool";
106			reusable;
107			size = <0x4000000>;
108			alignment = <0x2000>;
109			linux,cma-default;
110		};
111
112		display_reserved: framebuffer@78000000 {
113			reg = <0x78000000 0x800000>;
114		};
115
116		multimedia_reserved: multimedia@77000000 {
117			compatible = "acme,multimedia-memory";
118			reg = <0x77000000 0x4000000>;
119		};
120	};
121
122	/* ... */
123
124	fb0: video@12300000 {
125		memory-region = <&display_reserved>;
126		/* ... */
127	};
128
129	scaler: scaler@12500000 {
130		memory-region = <&multimedia_reserved>;
131		/* ... */
132	};
133
134	codec: codec@12600000 {
135		memory-region = <&multimedia_reserved>;
136		/* ... */
137	};
138};
139